Problem 22 · 2009 Math Kangaroo
Stretch
Logic & Word Problems
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In a haunted house the house ghost suddenly disappears. At that moment in time all clocks show 6:15. However, there is also one strange clock in the house that showed the correct time before that event — starting from the disappearance it begins to count backwards. At 19:30 in real time the house ghost reappears. What time does the odd clock show at that moment?
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Answer: A — 17:00
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Hint 1 of 2
The vanishing happens at 6:15 in the evening, that is 18:15.
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Hint 2 of 2
Find how much real time passes, then subtract it because the odd clock runs backwards.
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Approach: run the time backwards by the elapsed amount
- When the ghost vanishes the clocks read 6:15 p.m. = 18:15.
- The ghost returns at 19:30, so 1 hour 15 minutes of real time pass.
- The odd clock counts backwards, so it shows 18:15 − 1:15 = 17:00.
- The odd clock shows 17:00.
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